Low temperature pre-mixed combustion is known as a method of combustion for a diesel engine in order to reduce particulate matter, NO.sub.x emissions and the like which are discharged from the engine. The method entails retarding the fuel injection timing until after the compression top dead center, in addition, by reducing oxygen concentration by exhaust gas recirculation (EGR), the fuel ignition lag period is lengthened and a gaseous pre-mixed, in which the fuel is in a sufficiently gaseous state, is formed during the fuel ignition lag period. Thus low temperature pre-mixed combustion can be performed at low oxygen concentrations.
An arrangement which performs low temperature pre-mixed combustion even under high engine load by operating the engine at an engine compression ratio which is lower than the normal engine compression ratio, is disclosed in Tokkai-Hei-8-254134.
When the engine compression ratio is high, the compression temperature is relatively raised, so that the initial combustion temperature (the ambient temperature in the cylinder at combustion initiation) rises. Since the combustion temperature further rises after initiation of combustion, it is not possible to perform low temperature pre-mixed combustion when the permissible range for low temperature pre-mixed combustion is exceeded. In particular since the combustion temperature is relatively high when the engine is operating at high load, it has only been possible to perform low temperature pre-mixed combustion at low engine load.
When the load on the engine increases, the temperature of intake gases increases due to the increasing temperature of the recirculated exhaust gases. In addition the actual compression ratio increases due to the intake compression created by a turbo-charger. As a result, the temperature at the initiation of combustion exceeds the temperature range for low temperature pre-mixed combustion and as a result, low temperature pre-mixed combustion has only been possible at low loads.
In contrast, if the compression ratio is lowered, the initial combustion temperature is relatively lowered and low temperature pre-mixed combustion becomes possible at higher engine load.
However if the combustion ratio is lowered, the initial combustion temperature may fall below critical levels at low engine load which results in unstable combustion at low load.
As a result, the conventional engine above raises oxygen concentrations during low engine load at which low temperature pre-mixed combustion can not be performed by reducing the EGR amount. In addition, by advancing the fuel injection timing, combustion initiation is advanced so as to stabilize combustion. However in such a case, NO.sub.x emissions are increased due to the advanced fuel injection timing and the reductions in EGR. Furthermore combustion noise is increased and fuel efficiency is reduced due to the fact that low temperature pre-mixed combustion can not be performed.